This invention is directed to an improved wear-resistant conduit typically for use in plastic injection molding operations. In the plastics industry such devices are referred to as "sprue bushings." Sprue bushings serve as a conduit for transfer of molten plastic material from an injection molding machine into a mold. Sprue bushings are known in the art and are described in Applicant's U.S. Pat. No. 4,950,154.
A sprue bushing is typically inserted into a corresponding opening in a mold. The mold and sprue bushing are next positioned within a plastic injection molding machine. As the molding cycle takes place, the nozzle seat portion of the sprue bushing mates with a corresponding injection molding machine nozzle tip. Molten material is then discharged from the nozzle tip and through the sprue bushing. The molten material is discharged from the sprue bushing and into the mold filling the mold cavity producing a molded part. Following cooling of the plastic, the mold is opened and the part discharged into a drop box or the like.
The sprue bushing plays an important role in the molding cycle. In addition to serving as a conduit for channelling the molten material from the injection molding machine into the mold, the sprue bushing serves as a heat sink for removing heat from the molten plastic. This heat-transfer process decreases the time needed for cooling of the part before the mold can be opened and the part ejected.
Further, the sprue bushing aids in handling and processing of the molded part through formation of what is known in the industry as a "sprue." A sprue is the hardened plastic portion remaining within the sprue bushing passageway. The sprue forms a stem-like projection on the molded part or is part of the mold runner system.
The sprue is a highly desirable attachment point for automated machines used to sort the part or perform further processing. The sprue could not be used for such gripping purpose if the sprue was molten and soft. Rapid cooling of the plastic used to form the hardened sprue is greatly facilitated by the heat-transfer characteristics of the sprue bushing. Another advantage of rapid sprue cooling is that the sprue does not bond to other parts in the drop box thereby reducing product defects.
Prior art sprue bushings have several disadvantages. One important disadvantage is that the heat-conductive materials used to make the sprue bushings tend to be relatively soft and can degrade as abrasive materials are passed through them. For example, many injection-molded plastics include materials such as glass and talc which are highly abrasive. The wear caused by passing these materials through the bushing will result in irregular widening of the bushing's flow passageway. Uneven passageway wear can cause irregular material flow resulting in inconsistent and incomplete filling of the molds.
Another disadvantage is that excessive wear may cause contamination of the molded products. Contamination may occur when bushing body material is swept into the mold.
The service life of certain prior art sprue bushings may also be shortened particularly when such bushings are used with abrasive materials. For example, a prior art sprue bushing may have a service life of 4000 mold cycles when used with abrasive materials. The required replacement of the sprue bushing results in potentially unnecessary costs to the operator.
A sprue bushing which would make the injection molding process more efficient and cost effective by shortening cycle times, by producing a molded product capable of easy manipulation and processing with few product defects and which would have a long service life notwithstanding exposure to abrasive materials would represent an important advance in the art.